For the milling system employed in a conventional coal-fired power plant, a coal mill (routinely there are two types of coal mills: medium speed coal mills and fan-type coal mills) is used for milling and drying coal, into which is introduced hot flue gas at 150 to 300° C. from a coal-fired boiler economizer, and then blowing the resulting powder into a boiler and burning it. However, this method has such shortcoming that the degree of drying is so low that it is impossible to directly make high moisture lignite meet electricity generation requirements and moisture in the lignite cannot be recovered.
The drying system and the milling system employed in the above-mentioned conventional coal-fired power plant are two independent units, so the dried coal is transported to a coal bunker of the milling system via transportation equipment such as a scraper, a belt conveyor and a bucket elevator, etc. But there are problems as follows:
(1) The temperature of the dried pulverized coal ranges from 60 to 80° C., thus a large amount of dust and steam are produced during transport. Due to the major amount of fine powder and lower moisture content, a large amount of dust is easy to produce when a belt conveyor, a scraper or the like is used for transport. As a consequence, the environment is contaminated and the operating environment is relatively bad; besides, fine pulverized coal would spontaneously ignite very easily, and even a serious accident like flash explosion would occur, thereby affecting the stability of transportation equipment.
(2) The temperature of the hot coal, which has a temperature ranging from 60 to 80° C. after drying, is reduced to 30 to 60° C. through the step of transportation, and then the coal is fed into a coal mill and accordingly milled. The heat energy carried by the dried hot coal is wasted. According to calculation, the heat energy thus wasted accounts for 5 to 20% of the energy required by the milling system.
(3) The dry exhaust produced by coal drying is dedusted and then directly discharged into atmosphere. The heat and water vapor contained in the exhaust are not recycled.